This application relates generally to optical imaging. More specifically, this application relates to methods and systems for optical imaging and ranging.
In most situations, when dealing with recovering the three-dimensional structure of an object or a scene, the objects are opaque. Such is the case when viewing everyday objects such as a tool or a person. There are usually two variables of interest at each image pixel: the surface brightness (reflectance), and its distance from the camera (depth). These two variables (per pixel) are commonly recovered based on two input images.
There are several known methods to solve the problem of ranging and imaging. These include a variety of techniques for stereo imaging or triangulation in which the images are taken from different viewpoints. By measuring the disparity between the positions of corresponding image points, the distance of the corresponding object point is determined. One specific approach is described in U.S. Pat. No. 4,573,191, filed Mar. 29, 1984, entitled “Stereoscopic Vision System,” the entirety of which is herein incorporated by reference for all purposes.
Another approach is based on defocus blur. The image of an in-focus object point will be sharp while the image of a defocused object point will be blurred. The depth estimate of object points can be based on focus sensing with multiple images. This method is called depth from focus.
On the other hand, a more efficient approach is to estimate depth by comparing just two images taken with different settings of the imaging system. This method is termed depth from defocus. Using depth of field (DOF) cues, such as focus and defocus, are also preferable over stereo due to the higher numerical apertures used. This allows smaller diffraction problems and ultimately higher depth resolution. Nevertheless, diffraction limits the depth resolution when classical apertures are used. The depth resolution is determined by the DOF, which is a range of depths for which the blur kernel has no detectable change. Therefore, objects within the DOF will be assigned the same depth and no variations will be detected. This happens at the plane of focus, where the kernel changes slowly (quadratically) as a function of depth due to diffraction. Certain specific techniques are described in A. P. Pentland, “A New Sense For Depth Of Field,” IEEE Transactions on Pattern Analysis and Machine Intelligence, 9, 523 (1987), and M. Subbarao and Y. Liu, “Analysis Of Defocused Image Data For 3D Shape Recovery Using A Regularization Technique,” Proc. SPIE 3204, 24 (1997), the entirety of each of which is herein incorporated by reference for all purposes.
Still other methods for ranging use wavefront encoding as described in U.S. Pat. No. 5,521,695, filed Jun. 23, 1993, entitled “Range Estimation Apparatus And Method,” the entirety of which is herein incorporated by reference for all purposes.
None of these methods can achieve super-resolution beyond the depth of field, which is limited by the numerical aperture of the system. There is, accordingly, a need in the art for improved methods and systems for ranging and imaging.